Cambridge-MIT unveils 'silent' aircraft

SAX-40 - the future of airliner design?

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The Cambridge-MIT Institute yesterday unveiled its SAX-40 "silent aircraft" - the result of a £2.5m project launched back in 2004 to produce a vehicle "whose noise emissions would barely be heard above the background noise level in a typical built-up area".

The end result of the team's labours boasts a smooth "blended wing" design aimed at reducing noise caused by turbulent airflow. The SAX-40's tail-less aerofoil body provides sufficient extra lift to allow the designers to ditch conventional tail and flaps - both sources of irritating decibels. Landing gear is faired, once again cutting airflow-induced racket.

The wing concept demonstrates two further noise-tackling ideas: a retractable drooped leading edge which increases lift at low speeds and is attached to the main wing surface by a flexible material which prevents air passing between the two; and "trailing edge brushes" which smooth the transition of turbulent air passing over the wing and non-turbulent air behind it.

Engine design is, as promised by project manager Paul Collins back in 2004, based on conventional jet turbine technology, but with a difference. As the blurb explains:

Each engine has a single core, driving three high capacity low speed fans. This distributed propulsion system is designed to ingest the boundary layer on the aircraft centrebody which reduces the fuel burn. The multiple small fan design is easier to embed in the airframe, and leads to reduced weight and nacelle drag. It also enhances boundary layer ingestion, thereby improving fuel efficiency, and the low fan tip speeds lead to low noise.

These "GRANTA – 3401" powerplants also deploy "variable area exhaust" nozzles - allowing the aircraft to close the nozzles at cruise for increased fuel efficiency, and a vectored thrust capability to make sure the power's pointing in the most efficient direction for take-off and landing.

The upshot of all this is a much quiter aircraft ("63 dBA outside airport perimeter...some 25dB quieter than current aircraft" - more details here), burning 35 per cent less fuel than anything currently in the air. Indeed, the team reckons the SAX-40 will carry 215 passengers for 5,000 nautical miles, giving "149 passenger-miles per UK gallon of fuel (compared with about 120 for the best current aircraft in this range and size)". This, the boffins estimate, is equivalent to the Toyota Prius Hybrid car carrying two passengers.

All well and good, but the aircraft manufacturing industry will have to agree to a radical change in design direction to make the SAX-40 a reality. The technical challenges would be great, and the ongoing Airbus A380 tale of woe shows that even conventional airliner concepts can come seriously, and expensively, unstuck. ®